The Smith & Wesson (S&W) J-frame is the smallest class of its revolvers. S&W had two frame sizes in 1894 when it designed the side swing or swing-out-cylinders line of revolvers. The smaller-sized frame was called the “I-frame,” and the larger-sized frame was known as the “K-frame.” These designations were an internal way for employees to identify the frame sizes. However, S&W employees began using the internal frame designations in discussions with journalists, leading to their widespread use.
The J-frame was introduced in 1950. Ten years later, the smaller I-frame revolvers chambered for the less-powerful .38 S&W cartridge were discontinued. All small-frame guns made since have been built on the J-frame.